"GNS3 is an open source software that simulate complex networks while being as close as possible to the way real networks perform." This emulates a whole range of Cisco routers and similar devices -- very neat.

From userspace, it looks like TCP. To routers and NAT boxes, it looks like TCP. But it's actually using multiple TCP connections over different paths to improve reliability and throughput. The presentation is well worth a look, for a comprehensive overview of how your TCP connection is likely to get messed up by cheap/evil network hardware...